A spark plug is used as an ignition device which performs a spark discharge while using a gap between a center electrode and a ground electrode that are exposed in a combustion chamber of an internal combustion engine, as a spark discharge gap. Conventionally, in order to realize a prolonged lifetime of a spark plug, a tip of a noble metal alloy (hereinafter, also referred to merely as noble metal tip) which is excellent in spark wear resistance and resistance to oxidation at high temperatures (hereinafter, the spark wear resistance and the resistance to oxidation at high temperatures are often referred in combination as durability) is joined to a tip end portion(s) of one or both of the electrodes which will function as the starting point of a spark discharge (for example, see JP-A-8-339880). When a noble metal tip which is excellent in durability is used, it is possible to reduce the diameter of the noble metal tip which will function as the starting point of a spark discharge. Therefore, a configuration where the lifetime can be prolonged, and, when the tip is made very thin, spread of flame (hereinafter, also referred to merely as flame developing property) which is an ignited air-fuel mixture is not impeded can be realized (for example, see JP-A-2002-313524).
Before a very thin noble metal tip is used as means for improving the flame developing property, various improvements have been made because of the following reason. A spark plug is used while a male thread formed on a metal shell is fastened to a female thread formed in an internal combustion engine. In the mounting process, it is very difficult to mount a spark plug in consideration of the direction of the ground electrode. When an air-fuel mixture proceeds from the periphery of the spark plug to a place where a spark discharge occurs, and when the air-fuel mixture is ignited and then a flame develops, the movement of the air-fuel mixture or the development of the flame is blocked by the presence of the ground electrode, and hence there arises a problem in that the combustion efficiency is lowered.
In order to solve such a problem, a configuration which is disclosed in, for example, JP-A-11-121142, and in which a ground electrode has a circular section shape (hereinafter, such a ground electrode is referred to merely as round ground electrode) has been proposed. When this configuration is employed, a phenomenon that a flow of an air-fuel mixture is blocked by the ground electrode and a gas flow is separated from the ground electrode can be prevented from occurring, or another phenomenon that a flame in a growth process is contacted with the ground electrode, and hence heat is drawn from the flame, thereby impairing the development of the flame can be prevented from occurring.